1. Field of the Invention
This invention relates to novel pentamethine thiopyrylium salts, i.e. 2,6-di-t-butyl-4-[5-(2,6-di-t-butyl-4H-thiopyran-4-ylidene)penta-1,3-dieny l]thiopyrylium salts, a process for production thereof, and to photoconductive compositions containing them.
2. Description of the Prior Art
Thiopyrylium and pyrylium dyes are known to be useful in various applications. For example, they are used as an electron acceptor compound in direct positive photographic silver halide emulsions disclosed in Japanese Patent Publication No. 40900/71; or as spectral sensitizers for photoconductive materials, particularly spectral sensitizers for organic photoconductive materials, as described in Davis et al. U.S. Pat. No. 3,141,700, van Allan et al. U.S. Pat. No. 3,250,615, Reynolds et al. U.S. Pat. No. 3,938,994, and Research Disclosure No. 16321, page 5, November 1977.
Photoconductors sensitized with thiopyrylium and pyrylium dyes are used in the various applications disclosed in the above-cited patent documents, and are especially important for xerography or electrofax-type electrophotography.
The use of these known thiopyrylium dyes as sensitizers for photoconductive materials, however, cannot provide colorless, transparent photoconductive compositions because these dyes have an absorption band in the visible region.